In conventional electroless plating process, plating a tin layer onto surface of a copper pillar of a semiconductor substrate for preventing copper oxidation, then plating a silver layer for making the copper pillar is able to connect with the other semiconductor substrate. The time and cost of the process will be relatively higher due to perform tin-plating and silver-plating separately. Besides, the silver layer on the copper pillar of the semiconductor substrate is formed on the tin layer after tin plating process. Therefore, the coupling strength will be weaker when the semiconductor substrate connects with the other semiconductor substrate by the copper pillar.